This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The basic reactivity of sulfur-centered radicals is being studied in a variety of peptide/protein systems using Sulfur K-edge X-ray Absorption Spectroscopy (S K-edge XAS) as a novel probe of the electronic structure of these biologically-important peptidic radicals. Cysteinyl radicals ([CysS?]), their sulfoxyl derivatives ([CysSOn]?) disulfide anion radicals ([CysS?SCys]1-) have been observed or postulated as intermediates in several biological functions including enzymatic catalysis, long-range electron transfer, peptide post-translational modification, cellular redox buffering, and even cellular redox signaling. Methionine radicals ([MetS?]) are believed to be involved in neurodegenerative diseases pathways such as the formation of amyloid-beta plaques. Electron paramagnetic resonance is the primary tool used for characterization of radical species but this approach is complicated by spin orbit broadening in sulfur species. The S 3p